The party was the big winner in the 2003 election, running on the SP.A-Spirit joint list (cartel) with the social-liberal party Spirit. Their share of the vote went up from 9% (of the total Belgian vote) to almost 15%, a second place in the number of votes. The main victim of this resurgence was the Green! party (formerly known as Agalev). The sp.a was part of the "purple" federal coalitions of Prime Minister Guy Verhofstadt from 12 July 1999 until 10 June 2007, which contained both the Flemish and Francophone liberal and social-democratic parties.

In 2004, the sp.a along with its partner Spirit lost the elections for the Flemish Parliament. Although they won more seats in comparison to the Flemish elections of 1999, their percentage of the vote compared to the successful 2003 federal elections was considerably down. The reputation of then party leader Steve Stevaert took a beating too.

As of May 2009, sp.a was in opposition in federal politics; and unlike its Francophone counterpart the Socialist Party (PS), sp.a was not a participant in the Leterme II Government.

In January 2009 the party had apparently changed its name to Socialisten en Progressieven Anders. This name change was however retracted, and the party baseline instead was changed from "Sociaal Progressief Alternatief" to "Socialisten en Progressieven Anders".[11]

In the 2010 federal election sp.a won 13 seats with 9% of the overall vote. The party was a member of the Di Rupo Government formed on 6 December 2011, until the elections in 2014. In the elections for the Chamber of Representatives on the 25th May 2014, sp.a scored again 9% and received 13 seats, in contrast to their francophone Socialist Party counterparts, who lost 3 seats and whose share of the vote decreased by 2%. In the Flemish Parliament sp.a have 18 representatives, deriving from around 14% of the vote - this is a small reduction on the 2009 parliament, where sp.a had 19 seats, deriving from 15% of the popular vote. From 2009-2014, sp.a participated in the Flemish Government, in an uneasy coalition with the CD & V and the N-VA. From 2014 onwards, sp.a formed part of the opposition in Flanders, as the regional government reflected the Flemish component of the federal administration, consisting of coalition of the Open-VLD, CD & V and the N-VA.